The present invention relates to the preparation and use of novel electrostatic discharge materials. More particularly, the invention relates to the preparation of high molecular weight, linear polyurethane-ureas from diisocyanates, oxyethylene diols and water and the use of these materials in applications in the electrostatic discharge areas.
U.S. Pat. Nos. 3,852,255; 3,891,718; 8,893,979; and 3,911,047 disclose the synthesis of low molecular weight end-capped polyether polyurethanes for use as antistatic additives for fibers to provide surface resistivity values of the order of 10.sup.10 ohm after washing.
European Patent Application 0 142 792 discloses end-capped polyurethane oligomers useful for static dissipative coatings, providing 10.sup.12 -10.sup.13 ohm-cm volume resistivities.
U.S. Pat. No. 3,661,267 discloses using an aqueous bath to apply --CN groups to the surface of a preformed film or fiber to render it antistatic. U.S. Pat. No. 4,029,694 describes an ester/alkylene oxide antistatic agent for melt polymers such as the reaction product of dimethylterephthalate and an ethylene oxide adduct of a propylene oxide adduct of ethylenediamine. None of these references suggest the novel synthesis or polymers of the present invention.
It has long been known that static charges which develop between isolated bodies are discharged when those bodies are brought into sufficiently close proximity or contact. Potentials as high as 30,000 volts have been reportedly generated simply by a person walking on a synthetic carpet In recent years this phenomenon, always regarded as something of a nuisance, has become a major concern to the manufacturers of sensitive electronic equipment. A static discharge of only a few hundred volts can severely damage or ruin expensive electronic circuitry, and such damage can occur at any stage of the assembly process or during transportation or storage.
Static charges can accumulate on production workers, on assembly work surfaces, and on any of the tools and containers used in the assembly area. The need to prevent static discharge requires that the entire assembly environment be constructed from materials which will quickly dissipate static charge, effectively interconnecting all workers, surfaces and equipment with a common electrical ground.
There are two general classes of materials available for electrostatic discharge (ESD) protection in the electronics industries. The first of these classes is referred to as "conductive" materials, which typically have resistivities in the range of 10.sup.3 to 10.sup.6 ohms/square. These materials are typically made of plastics or elastomers which are filled or impregnated with conductive carbon black or metallic substances. In addition to their cost; most of these materials are dark and undecorative and cannot be coated for decoration, wearability or protection without losing their conductive qualities.
Conductive materials can also be made of inherently conductive polymers. Conductive polymers previously known include: polyacetylene, polyphenylene and poly(pyrrole). Dopants such as AsF.sub.5, substituted quinones, FeCl.sub.3, HClO.sub.4, BF.sub.4 and iodine are added to improve conductivity. However, stability and compatibility problems as well as high cost and limited availability has limited the applications for these known polymers.
Charge dissipative materials, the second class, typically have resistivities in the range of 10.sup.6 to 10.sup.11 ohms/square. Most of these materials achieve their electrical conductivities through the use of topical chemicals such as antistats. These chemicals do not normally conduct electricity themselves, instead, they absorb moisture from the air which provides conductivity. Typically antistatic chemicals are small in molecular size. When mixed with plastics, they migrate to the surface and provide conductivity, particularly through moisture absorption.
For these reasons, the previously known charge dissipative materials are sensitive to the humidity of the environment. Furthermore, the performance of the material tends to degrade over time as the antistat which migrates to the surface might be lost by evaporation, cleaning or contact with other objects.